1. Field of the Invention
The present invention relates to a heat-sensitive adhesive sheet which does not have adhesion or has very low adhesion at ordinary temperature, but exhibits adhesion at the time of heating and keeps its adhesion even after exhibiting adhesion.
2. Description of the Related Art
Adhesive sheets for labeling have been increasingly used in labels such as labels for pricing, labels for trade description (bar codes), labels for quality indication, labels for measure indication, and labels for advertisement. Information is recorded on these labels by ink-jet recording, thermal recording, pressure-sensitive recording, and other various recording procedures. Conventional adhesive sheets each with a release paper and an adhesive layer laminated on the side opposite to the information recording side of such labels have been widely used, since these adhesive sheets can be easily applied to an adherend only by removing the release paper and pressing the adhesive sheets onto the adherend. The release papers of these conventional adhesive sheets are removed before use, but the removed release papers are not recovered and recycled and are disposed in most cases. The adhesive layer (pressure-sensitive adhesive layer) has tackiness or adhesion and is very difficult to handle after removal of the release paper. For example, when an adhesive sheet is placed on an adhered accidentally and an attempt is made to remove the adhesive sheet in order to apply the same properly, the adhesive sheet becomes curled or wrinkled and breaks at worse.
Accordingly, heat-sensitive adhesive sheets that do not have adhesion at ordinary temperature and do not require release papers have received attention. Such heat-sensitive adhesives comprise a solid plasticizer and/or a liquid plasticizer and a thermoplastic resin emulsion as essential components and contain a tackifier and other additives. This type of heat-sensitive adhesives is applied to a side of a substrate opposite to a printed side and thereby yields a heat-sensitive adhesive material. The surface of an adhesive layer of the heat-sensitive adhesive material has no adhesion at ordinary temperature, but exhibits adhesion when heatted and keeps its adhesion for a while after removal of a heat source. In this type of heat-sensitive adhesive materials, the solid plasticizer first fuses and dissolves the thermoplastic resin and the tackifier to thereby exhibit adhesion. The liquid plasticizer is generally encapsulated in a microcapsule, and the shell (microcapsule) breaks or allows the liquid plasticizer to pass there through by heating, and the liquid plasticizer dissolves the thermoplastic resin emulsion and the tackifier to thereby exhibit adhesion, as in the solid plasticizer. The heat-sensitive adhesive materials do not require release papers and are advantageous from the points of resource saving and environmental protection, in contrast to the conventional adhesive materials.
In addition, the heat-sensitive adhesive materials do not exhibit adhesion under room temperature, therefore it easily comes off once it is accidentally applied on to the adherends. Moreover, it is advantageous in that once an accurate place is determined on the adherends, the heat-sensitive adhesive materials could be adhered only by adding heat thereon.
Some of such conventional heat-sensitive adhesive materials have some problems, although they have the advantages as above. One of these problems is adhesive strength thereof. Food plastic wrap for use in, for example, food point-of-sales (POS) systems has been changed from poly(vinyl chloride) wrap to polyolefin wrap to avoid production of dioxins and other environmental issues. POS labels are applied to such polyolefin wrap, but exhibit much lower adhesion and wettability with the polyolefin wrap than with the poly(vinyl chloride) wrap and are easily peeled off. Improvement in adhesion and wettability with the polyolefin wrap has been made on the regular adhesive materials comprising adhesives and release papers. An attempt to improve adhesion to the polyolefin wrap has also been made on the conventional heat-sensitive adhesive materials, but sufficient results have not yet been obtained.
Another one of the problems is decreased adhesive strength with respect to ambient and/or adherends at low temperatures. The heat-sensitive adhesive materials are used as POS labels for fresh food such as meat and fish that is refrigerated to keep its freshness. The heat-sensitive adhesive materials as the POS labels are applied to such refrigerated adherends as well as to adherends at low temperatures. However, the conventional heat-sensitive adhesive materials do not have sufficient adhesive strength to ambient and/or adherends at low temperatures. In addition, the POS labels are also applied to hot deli and other adherends and/or ambience at temperatures higher than ordinary temperature. Accordingly, the POS labels must be applied at temperatures over a wide range from low temperatures through ordinary temperature to high temperatures. However, the conventional heat-sensitive adhesive materials cannot be applied to ambience and adherends at temperatures over a wide range and are therefore unfriendly for use in, for example, supermarkets. Japanese Patent Application Laid-Open JP-A) No. 64-4681 discloses means of using a thermoplastic resin emulsion in which a resin having a lower Tg is covered with another resin having a higher Tg. However, the resulting thermoplastic resin composition does not exhibit high adhesion at temperatures over a wide range from low temperatures through ordinary temperature to high temperatures, since the two resins exhibit decreased compatibility (miscibility) with each other even after activation.
A third one of the problems is blocking. Blocking is a phenomenon in which adhesion or adhesive property is expressed where adhesion is not intended. Blocking may be induced, for example, when a heat-sensitive adhesive material is exposed to temperatures higher than ordinary temperature. Once blocking occurs, the heat-sensitive adhesive layer adheres to an outer surface (a side of the substrate opposite to the heat-sensitive adhesive layer) to thereby obstacle paper feed and adversely affect printing procedures on the outer side. This phenomenon occurs in either of the cases where the POS label is rolled and where it is cut to sheets and is stacked.
As solutions to prevent blocking, proposes are made on a process of adding a lubricant wax to a heat-sensitive adhesive (Japanese Patent Application Publication (JP-B) No. 62-21835), a process of adding an aliphatic metal salt to the heat-sensitive adhesive (JP-A No. 2000-73022), and a process in which the surface of a solid plasticizer is protected with an inorganic compound or a colloid particle to inhibit softening of the solid plasticizer to thereby prevent blocking (JP-A No. 06-57223, JP-A No. 06-100847, and JP-A No. 06-100848). However, the addition of wax does not sufficiently effectively prevent blocking but, instead, deteriorates adhesion. When the surface of the solid plasticizer is protected with an inorganic compound or colloid particle, it takes long for the solid plasticizer to fuse and to diffuse, and therefore it takes long for the resulting heat-sensitive adhesive to exhibit adhesion. Accordingly, the heat-sensitive adhesive exhibits deteriorated adhesion properties and is not suitable for practical use. Blocking can be mitigated by using a plasticizer having a high melting point, but the adhesive strength of the resulting heat-sensitive adhesive is significantly decreased. Accordingly, the heat-sensitive adhesive materials are transported and stored at such low temperatures as to avoid blocking under present circumstances.